Copyright © 2012 Pearson Education, Inc. Chapter 19 Chromosomes and Cell Division Betty McGuire Cornell University Lecture Presentation
Copyright © 2012 Pearson Education, Inc. Chromosomes and Cell Division Two types of cell division Form of chromosomes The cell cycle Mitosis: Creation of genetically identical diploid body cells Cytokinesis Karyotypes Meiosis: Creation of haploid gametes
Copyright © 2012 Pearson Education, Inc. Two Types of Cell Division The human life cycle has two types of cell division Meiosis Mitosis
Copyright © 2012 Pearson Education, Inc. Two Types of Cell Division Meiosis Gives rise to gametes that have half the number of chromosomes as the original cell In females Occurs in ovaries Produces eggs In males Occurs in testes Produces sperm
Copyright © 2012 Pearson Education, Inc. Two Types of Cell Division Mitosis Results in identical body cells Occurs during growth and repair
Copyright © 2012 Pearson Education, Inc. [INSERT FIGURE 19.1 ON THIS SLIDE]
Copyright © 2012 Pearson Education, Inc. Two Types of Cell Division Web Activity: The Human Life Cycle
Copyright © 2012 Pearson Education, Inc. Form of Chromosomes Chromosomes Location: nucleus Structure and function: each is a tightly coiled combination of a DNA molecule and specialized proteins called histones DNA contains genetic information, which directs body development and maintenance Histones help with support and control of gene activity
Copyright © 2012 Pearson Education, Inc. Form of Chromosomes Gene Specific segment of the DNA Directs synthesis of a protein, which plays a structural or functional role in the cell
Copyright © 2012 Pearson Education, Inc. Form of Chromosomes Somatic cells All cells except eggs and sperm In humans, have 46 chromosomes Two sets of 23 chromosomes One set of 23 from each parent
Copyright © 2012 Pearson Education, Inc. Form of Chromosomes Each somatic cell contains two chromosomes with genes for the same traits Called homologous pairs of chromosomes One chromosome of the pair is from the mother One chromosome of the pair is from the father
Copyright © 2012 Pearson Education, Inc. Form of Chromosomes Diploid A cell with two sets of chromosomes (2n) Genes also occur in pairs in diploid cells Members of each gene pair are located at the same position on homologous chromosomes
Copyright © 2012 Pearson Education, Inc. Form of Chromosomes Of the 23 pairs of chromosomes Sex chromosomes make up one pair Determine gender Two types: X and Y XX = genetic female XY = genetic male Autosomes make up 22 pairs Determine expression of most of a person’s inherited characteristics
Copyright © 2012 Pearson Education, Inc. The Cell Cycle Mitosis The process of cell division for body cells When one nucleus divides into two daughter nuclei with the same number and kinds of chromosomes
Copyright © 2012 Pearson Education, Inc. The Cell Cycle Cell cycle Predictable set of events that a cell goes through from its origin to its own division into two daughter cells Two major phases Interphase Cell division
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The Cell Cycle Interphase Period of growth and preparation for cell division; not a “resting period” Three phases G 1 S G 2
Copyright © 2012 Pearson Education, Inc. The Cell Cycle Interphase G 1 First “gap” Time of major growth before DNA synthesis begins Chromosomes consist of a strand of DNA and proteins
Copyright © 2012 Pearson Education, Inc. The Cell Cycle Interphase S DNA is replicated, and the two copies of the chromosome, called chromatids, remain attached at the centromere Two attached chromatids are genetically identical Called sister chromatids Growth continues
Copyright © 2012 Pearson Education, Inc. The Cell Cycle Interphase G 2 Second “gap” Period after DNA is synthesized and before mitosis begins Growth continues
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The Cell Cycle Cell division consists of two processes Mitosis Division of the nucleus Cytokinesis Division of the cytoplasm
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The Cell Cycle Developing embryo and fetus Body cells divide continually Children Body cells divide in growth and repair Adults Some cells lose ability to divide (most neurons) Some cells stop dividing but retain ability to divide should the need arise (liver cells) Some cells actively divide throughout life (skin cells)
Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells Mitosis occurs in four phases Prophase Metaphase Anaphase Telophase
Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells Prophase Chromatin condenses and forms chromosomes as DNA wraps around histones Nuclear membrane begins to break down Mitotic spindle forms
Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells Metaphase Chromosomes attach to mitotic spindles and form a line at the center of the cell Alignment ensures each daughter cell receives one chromatid from each of the 46 chromosomes
Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells Anaphase Sister chromatids of each chromosome begin to separate, splitting at centromere Now separate entities, the sister chromatids are considered chromosomes
Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells Telophase Nuclear envelope forms around each group of chromosomes at each pole Mitotic spindle disassembles
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Mitosis: Creation of Genetically Identical Diploid Body Cells Web Activity: Mitosis
Copyright © 2012 Pearson Education, Inc. Cytokinesis Cytokinesis Division of the cytoplasm Begins during telophase Ring of microfilaments contracts at midline of cell, eventually pinching cell in two
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Karyotypes Karyotype Constructed by arranging chromosomes from photographs based on size and centromere location Can be examined for defects in number and structure of chromosomes
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Meiosis: Creation of Haploid Gametes Gametes Differ from somatic cells Haploid (n) Have only one member of each homologous pair of chromosomes 23 chromosomes Formed by meiosis Two divisions that result in up to four haploid daughter cells
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Functions of meiosis Keeps the number of chromosomes in a body cell constant from generation to generation Increases genetic variability in population
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Meiosis involves two cell divisions Meiosis I – separates homologues Meiosis II – separates sister chromatids Each division has its own Prophase Metaphase Anaphase Telophase
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Meiosis: Creation of Haploid Gametes Meiosis I Preceded by interphase, during which each chromosome was copied and now consists of two attached chromatids Reduction division because it produces two haploid cells Each daughter cell has 23 chromosomes One member of each homologous pair Each chromosome consists of two sister chromatids
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Prophase I Chromosomes condense Homologous chromosomes pair Phenomenon called synapsis
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Metaphase I Matched homologous pairs line up at the midline of cell and attach to spindle fibers
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Anaphase I Homologous pairs of chromosomes separate and move to opposite ends of cell Each homologue still consists of two sister chromatids
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Telophase I Nuclear envelope forms around chromosomes at each end of cell Cytokinesis occurs to form two haploid cells
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Interkinesis Brief interphase-like period before meiosis II Differs from mitotic interphase in that there is no replication of DNA
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Meiosis II Prophase II Chromosomes condense Metaphase II Chromosomes line up along midline of cell
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Meiosis II (cont.) Anaphase II Centromere holding two sister chromatids separates Chromatids of each pair now considered chromosomes Chromosomes move to opposite poles of cell
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Meiosis II (cont.) Telophase II Nuclear membrane forms around chromosomes at each end of cell Cytokinesis occurs in both daughter cells, forming four haploid daughter cells
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes The important feature of meiosis II It separates the two sister chromatids of each chromosome
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Meiosis: Creation of Haploid Gametes Web Activity: Comparing Mitosis and Meiosis
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Following meiosis, changes in shape and function of the haploid cells result in functional gametes Spermatogenesis produces four sperm cells specialized for transporting the male’s genetic information to the egg Oogenesis produces up to three polar bodies and one ovum packed with nutrients to nourish the early embryo
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Meiosis: Creation of Haploid Gametes During meiosis, genetic variation is created through Crossing over Independent assortment
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Crossing over Occurs when corresponding pieces of chromatids of maternal and paternal homologues are exchanged during synapsis Prophase I
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Meiosis: Creation of Haploid Gametes Independent assortment The relative positioning of homologous maternal and paternal chromosomes with respect to poles of the cell is random Members of each homologous pair orient independently of other pairs Metaphase I
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Meiosis: Creation of Haploid Gametes Nondisjunction Failure of homologous chromosomes to separate during meiosis I or of sister chromatids to separate during meiosis II
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Nondisjunction results in too many, or too few, chromosomes in a cell The imbalance of chromosome numbers usually causes abnormalities in development and miscarriage
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Meiosis: Creation of Haploid Gametes Trisomy Three representatives of a chromosome in a cell Monosomy Only one representative of a chromosome in a cell
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Down syndrome An infant born with three copies of chromosome 21 Risk of having a baby with Down syndrome increases with maternal age Multiple physical and mental abnormalities
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Nondisjunction also can occur with sex chromosomes (X and Y) Nondisjunction of sex chromosomes during sperm formation Resulting sperm will carry both X and Y or no sex chromosome at all Nondisjunction of sex chromosomes during egg formation Resulting egg will have two X chromosomes or none at all
Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes Turner syndrome XO Klinefelter syndrome XXY
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